1. Field of the Invention
This invention relates to photoresist processes and compositions and is more particularly concerned with an improved process and compositions for the fabrication of microelectronic circuitry using photoresist systems.
2. Description of the Prior Art
The production of microelectronic circuitry, particularly metal oxide-semiconductor integrated circuits, has shown extremely rapid growth from medium scale integration to very large scale integration (VLSI) with rapidly increasing complexity of circuitry as well as vast increases in number of bytes per chip. One of the methods currently employed to achieve the fine-line photolithography required in VLSI is that known as the step-on-wafer process or the step-and-repeat alignment process; see, for example, E. C. Douglas, Solid State Technology, May, 1981 pp. 65-72; and E. W. Loebach, VLSI Wafer Printing with Step-and-Repeat Aligners, IGC Conference, Carmel, June 30-July 2, 1980. In this procedure a pattern of individual images is produced on a wafer covered with a first layer of photoresist. The individual images are projected and exposed using a reticle (photomask), optionally having an enlarged (usually up to about 10 times) version of the image to be reproduced, and employing a projection system with appropriate reduction lens if necessary. In the production of multiple layer images it is necessary to align each new image so that it is precisely superimposed on the corresponding images produced in a previous level. In order to facilitate such alignment of images a pattern of alignment markers is incorporated into each image in the first layer of photoresist and the reticle employed to reproduce the second and any subsequent levels of images is provided with a pattern of windows which corresponds to the pattern of markers in the first level images. By bringing the windows into correct alignment with the markers in the first level image it is possible to achieve accurate superimposition of the second and any subsequent images on the first one.
It has now been found that the alignment process can be greatly facilitated, and the possibility of mismatch of images greatly reduced, by the process of the invention which, in one of its broadest aspects, provides for incorporation of certain dyestuffs into the photoresist layers as will be more fully described hereinafter.
The incorporation of dyestuffs into photoresist compositions for other purposes has been described hitherto. Illustratively, Takahashi et al U.S. Pat. No. 4,356,254 teaches the incorporation of basic carbonium dyes into photoresist compositions used in production of printing plates in order to facilitate visual inspection of the developed image. No criticality is reported as to the particular wavelength in which the dyestuff must absorb or as to the relationshp between the wavelengths to which the photoresist and the dyestuff are sensitive.
Namiki et al U.S. Pat. No. 4,268,601 describes a process for producing photomasks in which a layer of photosensitive resin is applied to a polyvinylalcohol layer on a transparent support, optionally with a polyamide adhesive layer separating the resin from the polyvinylalcohol layer. After the photoimaging is carried out the combined layers are stripped from the support. Either the polyvinylalcohol layer or the photoresist layer can incorporate a pigment or dyestuff, the preferred embodiment having a dye in the polyvinylalcohol layer and a pigment in the photoresist layer. No criticality is attributed to the wavelength at which the dyestuff or pigment absorbs except that, if the wavelengths of absorption maxima for the colorant and the photoresist differ by not more than 5 namometers, it is recommended that the colorant be present only in the polyvinylalcohol layer and not in the photoresist layer. Where the difference exceeds this minimum the colorant can be present in either or both layers.
Peters et al U.S. Pat. No. 3,615,538 describes improving the physical properties of a photoresist image on a printing plate by incorporating a silane compound into the photoresist. Dyes or pigments can also be incorporated but no criticality is attributed to the absorption characteristics of the dye.
Gilson et al U.S. Pat. No. 3,647,447 teaches the coloring of photoresist images, particularly those derived from polycinnamates, using dyestuffs which have low absorption at 350-480 nm and are not soluble in the developer solution used in the imaging process.
Klupfel et al U.S. Pat. No. 4,241,166 describes incorporating into photoresist systems certain dyes which exhibit temporary decolorization of exposure to radiation used in the imaging process.
Sysak U.S. Pat. No. 4,341,860 teaches photoresist compositions which contain a leuco dye and a cyclohexadienone compound which serves to oxidize the leuco dye upon exposure to image-forming radiation.
Chen et al U.S. Pat. No. 4,362,809 uses a dyestuff to reduce problems in photoimaging caused by reflection of radiation from a substrate-photoresist interface. A two layer photoresist is provided on a substrate, the bottom layer incorporating a dyestuff which will absorb radiation at the wavelength used to image the top layer.
It has not been suggested previously that accuracy of alignment in the step-and-repeat alignment process, hereinafter referred to as the "stepper" process, can be greatly improved and facilitated by employing photoresist systems in association with dyes which meet certain very specific criteria. Nor has it been suggested previously that certain other advantages, to be described hereinafter, flow from the use of such photoresist systems in association with said dyes.